Nonvolatile memory devices have garnered much interest in recent times, including nonvolatile memory devices made of a phase changeable structure. Referencing FIG. 1, a phase changeable structure typically includes a lower electrode 200, a phase changeable pattern 310 and an upper electrode 410. A phase changeable pattern can be obtained from etching a phase changeable layer using a mask pattern. The phase changeable pattern is typically situated between the lower electrode and the upper electrode and can include a calcogenide.
Typically, a predetermined amount of current (e.g., generated by a difference in voltage between the lower electrode and the upper electrode) can be applied to the phase changeable pattern to induce a change in phase from a single crystalline state having a relatively low electric resistance into an amorphous state having a relatively high electric resistance. In addition, when the current applied to the phase changeable pattern is reduced or removed, the phase changeable pattern may be changed from the amorphous state into the single crystalline state.
In accordance with a conventional method of forming a phase changeable structure, a phase changeable layer is formed on a lower electrode. A metal nitride layer is then formed on the phase changeable layer. Thereafter, a first etching process is performed on the metal nitride layer to form an upper electrode 410 on the phase changeable layer, as illustrated in FIG. 1. A second etching process is then performed on the phase changeable layer to form a phase changeable pattern 310, as illustrated in FIG. 1.
When either or both of the first etching process or the second etching process is performed using a chlorine-containing etching material, a commonly used etchant, a significant amount of defects were detected in the phase changeable pattern. It is believed that chlorine may reside on a side face of the phase changeable pattern or at an interface between the phase changeable pattern and the upper electrode. It is also believed that this remaining chlorine residue or a byproduct thereof may contribute to defect formation, such as via erosion, in succeeding processes. An illustration of the typical defects that can be formed is illustrated in FIG. 1. Defects can occur between the phase changeable pattern 310 and the top electrode 410 as illustrated by reference number 302 in either or both portions of the phase changeable structure. Defects can also occur on a side face of the phase changeable pattern 310, as illustrated by reference numbers 304 and 306. Defects can also occur between phase changeable pattern 310 and the insulating layer, as illustrated by reference number 306. When such defects occur, erratic and discontinuous resistance to current behavior can be exhibited, as illustrated in FIG. 2.